Field of the Invention
The present invention relates generally to optical fiber outputs and, more specifically, to controlling the phase and amplitude of a light beam profile exiting an optical fiber.
Description of the Prior Art
A typical optical system will transmit, reflect, refract or otherwise modify the propagation of light or its salient properties such as phase, amplitude or polarization. In particular, an optical fiber will present at the cross-section of the output aperture a beam of light characterized by a certain amplitude (intensity) and phase distribution. The very familiar situation is that of the light propagation through a single-mode fiber which will have at the output a profile close to that of a Gaussian beam. The intensity is highest at the center and then it decreases as radius increases. The Gaussian beams are important because they maintain a Gaussian intensity profile at any location along the beam axis, even after passing through lenses (ignoring lens aberrations). The phase profile of such a beam is also very simple, usually linear or quadratic (described by a polynomial). The quadratic case is important as it is implying convergence or divergence of the beam (change in the beam radius).
There are however many situations when a Gaussian beam is not desirable. Particle trapping and ultra high-resolution fluorescence microscopy are achieved using beams that have a ring or doughnut shape (no light in the center). Flat top beams, where the intensity is constant over most of the cross-section, are also of interest when uniform illumination and efficient focusing are required such as in material laser processing. Most of the work is done in bulk, with light beams manipulated by macro optics (gratings, phase plates etc.).
Beam shaping can be implemented through different techniques: use of apertures, use of a combination of various optical elements, such as micro-lens arrays, or through manipulation of the near field which results in the desired changes in the far field. This last method, requiring modification in the near field of the beam phase rather than amplitude, is easy to implement. It can be achieved by placing a phase mask in the beam path. It also provides the desired profile with minimal loss in total energy. In very few cases direct beam manipulation was performed at the output of an optical fiber.
Beam shaping has been researched intensively and a variety of patents have provided a multitude of approaches. For example, U.S. Pat. No. 8,031,414 (2011), U.S. Pat. No. 8,016,449 (2011), and U.S. Pat. No. 7,593,615 (2010) provide for instructive reading with respect to various means of beam shaping (all covering refractive methods using external lenses, diffusers, waveguides or other optical elements). Prior art discussing the idea of creating a phase mask-like structure directly on the fiber end is extremely limited. Existing approaches require deposition of photosensitive material on the fiber end, material in which the surface structure is to be created.